I am trying to connect the 3.3V MMA8452Q Accelerometer (on BoB) to the Arduino Pro Mini (5V) and haven’t been able to establish communication.
I have also tested the accelerometer with the Arduino Uno (R3) with the same results.
Links to products being used:
MMA8452Q BoB: http://www.sparkfun.com/products/10955
Arduino Pro Mini: http://www.sparkfun.com/products/9218
Arduino Uno (R3): http://www.sparkfun.com/products/11021
Wiring (to Pro Mini):
Because the MMA pins are not 5V tolerant I added voltage dividers (10k/22k) on all four communication pins (SDA, SCL, INT1, & INT2) as well as on the supply pin for the BoB. This resistor combination drops the voltage to ~3.43v, within the 3.6v maximum for the chip.
I tested using only the internal 10k pull-up resistors on the BoB, as well as adding external pull-ups (4k7).
All other wiring matched the example sketch linked at the bottom of the accelerometer product page:
http://dlnmh9ip6v2uc.cloudfront.net/dat … xample.zip
And, yes, just in case your wondering, I used the A4 & A5 labeled on the bottom side of the board (the SDA & SCL pins per the Arduino schematics).
(Ok, ok, I did connect them to the wrong A4 & A5 the first time… but I fixed it!)
The code I tried was the example code provided above. The only change I made was changing the SA0 value from 0 to 1 (and back) hoping that would fix the problem. My second fix (though it should have been the first) was to double check the baud rates. Correcting the baud rates got me a single output (“Could not connect to MMA8452Q: 0xFF”) instead of garbage. (This error message is part of the example sketch, in case that wasn’t obvious).
My theories on why this may not have worked are 1) I somehow fried the board (ESD or missed a wiring issue?) or 2) The board is DOA (less likely) or 3) Something with the voltage dividers is screwing with communication (eg 5V drops to 3.4V one way and drops again down to ~2.4V the other way). or there is always 4) I’m a freaking idiot and missed something obvious.
The voltage dividers shouldn’t be the issue since (supposedly according to another post) the arduino will recognize voltages of less than 1V as high, but that was my first thought before I started researching heavily.
I found it interesting that the example code didn’t mention using voltage dividers or logic level shifting, though it does mention not using the wire.c library in order to avoid the Arduino’s internal pull-ups to 5V. This made me wonder if it was written for a specific board?
I also found part of the example sketch wording slightly confusing (though this is more of a side note than a reason the communication isn’t working).
SDA and SCL should have external pull-up resistors (to 3.3V). 10k resistors worked for me. They should be on the breakout board.
This sketch is dated 4 days prior to the BoB schematics being updated to add on-board 10k pull-up resistors. In my research (I don’t remember if it was on the MMA8452Q or general I2C communication) there was mention of the pull-up resistor values effecting the sample rate or communication frequency. How does this play into things?
I also ran across a lot of conflicting information (use logic level shifting, don’t use shifting it only complicates it, etc. etc.). I did find this post (http://www.arduino.cc/cgi-bin/yabb2/YaB … 1275738825) saying that modifying the wire library to not use the internal pull-ups allows 3.3v communication from the Arduino, but the discussion was specifically about the Arduino Duemilanove Atmega328. Is this the same for all Atmega328 based Arduinos?
My test with the Arduino Uno (R3) also yielded no communication. This time I followed the example code wiring without any voltage dividers (mistake?). I kept the testing time short just in case, but that part still made me nervous. I also tested this with and without the external pull-ups to 3.3V as well as SA0 values of 1 and 0.
Any tips, pointers or suggestions you can offer would be greatly appreciated. If more information is needed just let me know and I will be happy to provide as much as I can.
Thanks
The issue I see with this is that I2C has a bi-directional line(SDA). Since it is bidirectional, both sides send data from that line. The voltage divider setup will work when going from the Mega to the BoB. But not back, because all of the current gets sucked down through the 10k resistor. (Analogy: Its hard swimming up a waterfall.(Its not that good, just get over it.)). So the Mega sees no voltage. So no data back. The Mega is lonely. No one to talk to. 